Radio Frequency (RE) shielding is required on certain semiconductor devices in order to minimize Electro-Magnetic Interference (EMI) radiation from the semiconductor device. RE shielding is further required to prevent RF radiation from external sources from interfering with operation of the semiconductor device.
RF shielding is generally accomplished in one of three ways. A first method is to attach a metal can over the component after the component is attached to the motherboard. However, shield attach on the mother board has several problems. First, shield attach can be costly and a low yielding process. External shields soldered to the motherboard further require additional board space.
An alternative to the shield attached method described above is an embedded RF shield. In an embedded shield, the metal RF shield is directly attached to the semiconductor package substrate by means of solder or a conductive adhesive. The shield may be fully embedded within the mold compound of the finished package or can be exposed after assembly. In either case, the addition of a metal shield as a component attached to the top surface of the substrate is problematic for several reasons. First, the addition of a metal shield as a component attached to the top surface of the substrate requires a significant amount of additional space on the package substrate and adds additional thickness to the package. Second, it can be difficult to transfer mold in and around the metal shield to fully encapsulate the semiconductor package. Shield attach is also problematic due to flux creep during shield attach which may cause delamination and extrusion issues
The third method is the conventional conformal shield. In this method, all of the components are placed on the substrate and the substrate, or strip, is over-molded using unit molding, or pin gate molding where individual mold caps are defined within the strip such that upward facing, exposed pads in the substrate remain exposed after the mold operation. A conductive material is then applied to the strip such that it covers the units and also makes electrical contact to the upward facing pads. The strip is then singulated into individual units. While this technique eliminates the molding process concerns associated with the aforementioned embedded shield method, it does not eliminate the added substrate size required to form the so-called upward facing, exposed pads.
POP packages create a problem for RF shielding. For a PoP package, the ground connection for the RF shield must be electrically isolated from functional ground signals for the base/bottom and top PoP packages. In order to meet this requirement, the PoP base package must be shielded on the top and 4 sides of the package, while leaving contacts on the base package exposed and available for mounting the top PoP package. In addition, there can be no possibility to short the shield material to the PoP solder joint.
Therefore, a need existed to provide a system and method to overcome the above problem. The system and method would provide an RF shield for a PoP base package which leaves contacts on the base package exposed and available for mounting a top PoP package while reducing the possibility of shorting the shield material to a PoP solder joint.